Flooding at the Cosumnes River Preserve on February 14, 2017 along Franklin Blvd. in the lower Cosumnes River Watershed. Source: Tom Palmer

by Michelaina Johnson

This winter’s barrage of rain storms has driven most Central Valley rivers to the point of near record-breaking flooding, and the Cosumnes River is no exception. On February 10th, the Cosumnes hit the second highest peak flow in its recorded flow history: 45,400 cubic feet per second at Michigan Bar.

The Cosumnes is the only river draining the western Sierra Nevada Mountains that has no major dam. As a result, the river’s natural floods have limited – but not eliminated – development and agriculture in the Cosumnes’ lower watershed while also sustaining some of the best native habitat remaining in the Central Valley. Today, the 50,000+ acre Cosumnes River Preserve (CRP) protects the lower watershed’s exceptional habitat using a unique public-private partnership model that weaves together the seemingly conflicting human and environmental demands of the landscape – ultimately benefitting both.

When the earliest settlers came to the lower Cosumnes River watershed in the late 1840s and early 1850s, they found a landscape that was far different from today’s. The ranches and crop fields were once a marshland filled with tules and willow thickets. The Plains Miwok managed the landscape through controlled burning and hunting, among other practices. Even the Cosumnes’ acclaimed oak riparian forest stands did not exist in the lower watershed a century and a half ago.

Early settlers diked and drained the lower watershed, which converted the marshland into productive farm and grazing land in just a few decades. By the late 1870s, agriculture and cattle dominated the region. Due to this extensive reclamation and the damming of most of the Central Valley’s rivers, the Valley lost nearly all of its native habitat with a few notable exceptions, such as the Cosumnes River watershed.

Between 1900 and 1974, there were five distinct water projects totaling more than 30 dams proposed for the Cosumnes River. The most notable one was the Cosumnes River Project, which was part of the Central Valley Project (CVP) and called for the construction of six dams and reservoirs on the Cosumnes River, the largest being a 900,000 acre-foot reservoir on the Cosumnes’ main stem, at a cost of $180 million. Each of the proposals failed due to high costs, the inability to secure water rights, and farmer resistance, among other reasons.

The absence of a major dam on the Cosumnes River meant that the river flooded nearly every year, which relegated farmers to growing annual crops and ranchers to grazing livestock. These two land uses happened to be compatible with waterfowl and some native habitat types like vernal pools and riparian forests. Over the course of a century and a half, farming and ranching activities worked to unintentionally preserve these flora and fauna.

Even though vernal pools did not historically evolve with extensive grazing, two studies found that vernal pool landscapes in the Sacramento Valley have come to depend on this land use (Barry, 1998; Marty, 2005). The lower watershed has a long history of grazing dating back to the late 1840s, which, due to the river’s frequent flooding, remained a driving force behind the unintentional preservation of a significant percentage of vernal pools remaining in the Central Valley. The CRP today protects 14,141 acres of vernal pools, which is about ten percent of the total 137,100 acres remaining in the Central Valley.

Perhaps more surprisingly, cattle grazing also partly enabled the growth of riparian forest in the lower watershed where it didn’t “naturally” occur before reclamation. Limited coring of the remnant forests on the Preserve suggests that the oldest forest stands appeared around the turn of the twentieth century, only decades after the diking and draining of the lower watershed’s marshland was completed. As other watersheds suffered reclamation and damming, California lost 90% of the estimated 922,000 to 1.6 million acres of its historic riparian forests. But in the Cosumnes, the combination of reclaimed land and annual floodwaters created the environmental conditions necessary for riparian forests to take root and grow. Cattle grazing along the riparian corridor ensured that only the understory rather than the trees were consumed, leaving the largest oak riparian forest remaining in California in the lower Cosumnes River watershed.

Riparian forest in the lower watershed flooded from January’s strong rainstorms. Source: Mike Eaton.

As well as promoting new habitats, the reconciled use of the Cosumnes River watershed also supports long-present species. Unlike the riparian forest stands in the lower watershed, waterfowl have resided in the region, as well as the rest of the Central Valley, for more than a million years. The birds stop over in the Central Valley during their annual winter migration to fuel up for their journey to the wetlands of northern Alaska and western Canada. When early settlers converted 90% of the Central Valley’s original four million acres of seasonal and perennial wetlands to other land uses, principally agriculture, beginning in the early 1850s, the waterfowl turned to whatever food sources became available, including rice, alfalfa, wheat, corn, and pasture land. Due to the Cosumnes’ annual flooding, farmers opted to grow crops that could be planted and harvested before the unregulated deluge could destroy their agricultural investment. These annual crops, specifically wheat and corn, happened to serve as food sources for waterfowl –  much to the chagrin of farmers, some of whom shot the birds to keep them away from the fields. Since the Preserve’s founding in 1987, more than 200 species of birds have been observed within the preserve’s boundaries, including the state listed threatened Greater Sandhill Crane.

The Nature Conservancy (TNC) became interested in the Cosumnes’ riparian forest stands in the early 1980s and decided to establish the Cosumnes River Preserve in 1987. TNC, along with its myriad partners, quickly realized that other rare habitat types and bird species resided in the lower watershed and capitalized on their presence to create a cutting-edge conservation model for the Delta. The Preserve uses conservation easements to ensure that the historic, wildlife compatible land uses present on 21,271 acres, or 46% of the Preserve’s property, continue in perpetuity. The CRP also leases preserve land to farmers and ranchers to allow them to cultivate annual crops that can also function as food sources for waterfowl.       

The Cosumnes River Preserve today protects the Cosumnes’ free flow, floodplain, and rare native habitats. Through experimental, cutting-edge restoration techniques, such as levee breaching, the Preserve, in partnership with researchers from UC Davis, has shown that the restoration of the floodplain will lead to the regrowth of riparian forests, increase aquatic and avian wildlife habitat, and improve groundwater recharge through reconnection with the floodplain – all while remaining compatible with agriculture. This type of restoration has made the lower Cosumnes River watershed into an exemplary working floodplain. As a result of the Cosumnes River Preserve’s innovative public-private partnership management model and restoration techniques, state policymakers today look to the Cosumnes River as a restoration model for other rivers in the Central Valley and for the Delta.

Michelaina Johnson is a senior at UC Berkeley majoring in History and double minoring in Spanish and Conservation and Resource Studies. She completed her senior honors thesis, “Evading Dam-Nation: Land Use History of the Lower Cosumnes River Watershed, ca. 1820-2016,” last December and based this article on her research. She plans to pursue a career in either environmental journalism or water policy after graduating. 

Further Reading:

Ahearn, Dylan S, Richard W Sheibley, Randy A Dahlgren, and Kaylene E Keller. “Temporal Dynamics of Stream Water Chemistry in the Last Free-Flowing River Draining the Western Sierra Nevada, California.” Journal of Hydrology 295, no. 1–4 (2004): 47–63.

Barry, Sheila J. “Managing the Sacramento Valley Vernal Pool Landscape to Sustain the Native Flora.” In Ecology, Conservation, and Management of Vernal Pool Ecosystems – Proceedings from a 1996 Conference, edited by C.W. Witham, E.T. Bauder, D. Belk, W.R. Ferren Jr., and R. Ornduff, 236-240. Sacramento: California Native Plant Society, 1998.

Bennett, Karen Louise. “The River That Got Away: An Investigation into the Proposed Development Projects, the Players and the Political Climate That Helped Shape the Fate of the Cosumnes River.” Master’s Thesis, California State University, Sacramento, 1997.

Marty, Jaymee T. “Effects of Cattle Grazing on Diversity in Ephemeral Wetlands.” Conservation Biology 19, no. 5 (2005): 1626-1627.

Robinson, April, Samuel Safran, and Julie Beagle, “A Delta Renewed: A Guide to Science-Based Ecological Restoration in the Sacramento-San Joaquin,” San Francisco Estuary Institute—Aquatic Science Center, a report of SFEI-ASC’s Resilient Landscapes Program, publication #799, Richmond, CA, 2016.

Steding, Anna. “Restoring Riparian Forests and Natural Flood Regimes: The Cosumnes River Preserve.” In Sustainable Use of Water: California Success Stories, edited by Lisa Owens-Viani, Arlene K. Wong, and Peter H. Gleick, 229-239. Oakland: Pacific Institute for Studies in Development, Environment, and Society, 1999.

Whipple AA, Grossinger RM, Rankin D, Stanford B, Askevold RA. “Sacramento-San Joaquin Delta Historical Ecology Investigation: Exploring Pattern and Process.” Prepared for the California Department of Fish and Game and Ecosystem Restoration Program. A report of SFEI-ASC’s Historical Ecology Program, publication #672, San Francisco-Estuary Institute-Aquatic Science Center, Richmond, CA, 2012.